Amorphous tensile fracture strength

Fig. 16. Correlation between Young s modulus ( ) and T, tensile fracture strength (o,) or //, for Al-Ni-Zr amorphous alloys.

On the other hand, if an amorphous polymer is struck above the Tg, i.e. in the rubbery state, large extensions are possible before fracture occurs and, although the tensile strength will be much lower, the energy to break (viz. the area under the curve) will be much more, so that for many purposes the material will be regarded as tough. 

Figure 5 presents the results of tensile tests for the HPC/OSL blends prepared by solvent-casting and extrusion. All of the fabrication methods result in a tremendous increase in modulus up to a lignin content of ca. 15 wt.%. This can be attributed to the Tg elevation of the amorphous HPC/OSL phase leading to increasingly glassy response. Of particular interest is the tensile strength of these materials. As is shown, there is essentially no improvement in this parameter for the solvent cast blends, but a tremendous increase is observed for the injection molded blend. Qualitatively, this behavior is best modeled by the presence of oriented chains, or mesophase superstructure, dispersed in an amorphous matrix comprised of the compatible HPC/OSL component. The presence of this fibrous structure in the injection molded samples is confirmed by SEM analysis of the freeze-fracture surface (Figure 6). This structure is not present in the solvent cast blends, although evidence of globular domains remain in both of these blends appearing somewhat more coalesced in the pyridine cast material.

Since whiskers have high tensile strengths they are also capable of withstanding exceptionally large elastic strains. Metallic and even some oxide whiskers support strains of 2 to 5% before fracture or yield occurs. Towards the higher strains the stress-strain behavior is often nonlinear and substantial deviations from Hooke s law are observed. The stress-strain curves are similar to the one shown in Fig. 45 for the amorphous iron alloy fiber. At the highest strain some stress relaxation may also occur, giving rise to an irreversible residual deformation. 

As previously stated in Chapter 3, isotaetie polypropylene is a polymorphic material with four basic crystalline forms, namely the monoelinie (a), trigonal (P), orthorhombic (y) and mesomorphic smectic (intermediate state between ordered and amorphous phase) forms. The a-form exhibits excellent modulus and tensile strength but poor fracture toughness. ... 

Polymer composites have a fracture toughness, ultimate tensile strength and ultimate elongation that can be varied by changing the glass transition temperature of the composite. The thermodynamic properties of the amorphous matrix change at Tg. 

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